Conventionally, as an apparatus for efficiently forming a thin film on a large-area substrate to be processed in a sputtering method, there is known a sputtering apparatus in which a plurality of targets are disposed in parallel with each other in a manner to lie opposite to the substrate to be processed, inside a vacuum chamber. Among these targets disposed in parallel with each other, plural sets of bipolar pulsed power supplies are assigned to respective pairs of targets. While each of the bipolar pulsed power supplies is operated in synchronization, power is supplied (outputted) to each of the targets by alternately changing the polarity. Each of the targets is thus switched alternately between anode electrode and cathode electrode. Glow discharge is caused to be generated between the anode electrode and the cathode electrode to thereby form a plasma atmosphere so that each of the targets is sputtered (patent document 1).
Generally, the bipolar pulsed power supply is constituted by a rectifying circuit which supplies DC current, and a MOSFET bridge circuit which is connected to the positive and the negative output ends of the rectifying circuit and which is made up of four switching elements. Each of the switching elements is appropriately operated by the control means to charge pulse voltage at a predetermined frequency to the targets that make a pair. According to this arrangement, there is an advantage in that the electric charges to be accumulated on the surface of the targets are cancelled when an opposite phase voltage is charged, thereby obtaining a stable discharge.    Patent Document: JP-A-2005-290550